1. Field of the Invention
The invention relates to a process that combines a simultaneous saccharification and fermentation (SSF) process and a simultaneous fermentation and isomerization of xylose (SFIX) process to provide a simpler and reduced cost process for producing ethanol. In particular, the invention pertains to an improved process for producing ethanol from a mixed stream of xylose and cellulose and includes fermenting the mixed stream using a cellulase and xylose isomerase enzyme, under predetermined conditions, and includes simultaneously saccharifying the cellulose while fermenting the soluble sugars produced, and concurrently isomerizing the xylose while fermenting the xylulose as it is produced.
2. Description of the Prior Art
U.S. Pat. No. 4,663,284 to Jeffries discloses a process for producing ethanol from D-xylose by fermentation with xylose metabolizing yeasts, wherein small quantities of glucose are added to the fermentation medium during the fermentation process; however, the process is not an enzyme mediated process. Yeast strains can ferment xylose if oxygen is allowed to be present in the fermentation. The process of Jeffries further disclose that the addition of glucose to these oxygen mediated fermentations improves the yield of the fermentation; however, cellulose fermentation is not taught or included.
U.S. Pat. No. 4,511,656 to Gong pertains to a method for producing ethanol directly from D-xylose through fermentation of D-xylose by yeast mutants. The process further provides for directly and simultaneously obtaining ethanol from a mixture of cellulose and hemicellulose through yeast fermentation of D-glucose and D-xylose; however, these are oxygen mediated fermentations of xylose and are supplemented by the addition of glucose, as the sugar or as a hydrolysate containing glucose. In addition to oxygen being required, no enzymes are used and cellulose is not fermented.
In U.S. Pat. No. 4,490,468 to Gong et al., there is described an anaerobic fermentation of xylulose previously obtained by isomerization of xylose, and example 6 thereof briefly mentions the possibility of simultaneous isomerization and fermentation of xylose; however, the process is not combined in any way with the fermentation of cellulose.
U.S. Pat. No. 4,368,268 to Gong relates to a process for the production of ethanol from xylulose. The process includes isomerizing the xylose to xylulose and fermenting the xylulose to ethanol. Essentially, this process is the fermentation of xylose or xylose and other sugars in hemicellulose hydrolysates by mutant strains of yeast, either aerobically or anaerobically; however, hemicellulose does not refer to cellulose but to extracts obtained by pretreatment of materials that contain cellulose. The sugars obtained are soluble sugars (in most cases mostly xylose). Cellulose is not soluble and must be enzymatically digested to produce soluble sugars. Further, fermentations of xylose or hemicellulose, whether or not the hemicellulose contains some glucose, is not cellulose fermentation, and the fermentations in this patent are not anaerobic but oxygen mediated rather than enzyme mediated fermentations.
U.S. Pat. No. 4,840,903 to Wu discloses a process for the production of ethanol by a fungal strain capable of slowly degrading and fermenting cellulose, xylose, and a number of other sugars. Like simultaneous saccharification and fermentation (SSF) of cellulose, cellulase enzymes were added to the fermentations to produce glucose from cellulose; however, the fermentations are not a combination of enzymatic isomerization of xylose to xylulose a fungal strain rather than ethanol tolerant yeast was used for the fermentation, and fungal strains take much longer to grow and ferment, and these longer lengths of time or slow rates are unacceptable for industrial purposes.
In biomass materials, cellulose and hemicellulose are the two most abundant and renewable raw organic compounds, and together they compose above 70 percent of the entire world's plant biomass on a dry weight basis. These raw materials are widely available in waste from agricultural, forest, vegetable and food process sources, and the efficient recycling of these wastes to useful products, such as ethanol, would help to reduce disposal problems as well as provide an abundant and cheap source of fuel.